The present invention relates to satellite communication systems and communication methods therefor and, more particularly, to such systems and methods by which substantial improvements in overall communications efficiency are obtained.
In active communications satellite systems, a transponder-equipped satellite is positioned in a geostationary orbit to provide broadcast, navigation, communications, or similar services to a service area or areas. A service area is defined as the geographic region in which an earth station can receive signals from or send signals to the satellite economically. Various transponder architectures are available for communication satellites, one common architecture includes a broad-area beam antenna that covers the service area or areas for receiving up-link transmissions across an entire allocated up-link frequency band (e.g., 14.0 to 14.5 GHz) and a receiver that amplifies and down-converts the up-link transmission frequencies to frequencies within an allocated down-link frequency band (e.g., 11.7 to 12.2 GHz). Band-pass filters coupled to the receiver output divide the down-converted frequency spectrum into separate channels of selected bandwidth (e.g., 40 MHz). Each channel is amplified along separate gain paths by one or more stages of amplification that include a high power amplifier (HPA), typically, a traveling wave tube amplifier (TWTA) as the final stage. After post-amplification signal processing that can include additional filtering and combining, the HPA amplified signal energy is presented to the broad-area beam antenna for down link transmission to the service area or areas. In order to optimize the efficiency of the post-receiver amplifier function, the high power amplifiers are oftentimes operated in a non-linear mode, that is, the operating region where additional input signal power does PG,3 not cause a proportional increase in output signal power. When operating in this mode, the HPA will generally amplify one signal carrier most efficiently; multiple signal carrier amplification causing intermodulation interference and other problems.
Various up-link and down-link arrangements have been used to effect communications with the satellite. In particular, discrete frequency bands have been allocated on an international basis for up-link and down-link transmission. Because of orbit/spectrum scarcity, as well as economic reasons, it is desirable to increase spectral efficiency by reusing the frequencies in the same satellite or orbit location. For example, the same up-link and/or down-link frequency can be used simultaneously in two separate narrow-area beams directed from the satellite to widely separated geographical locations on the earth, the angular separation between the two beams preventing mutual interference between the two in both the up-link and down-link signals. It is also known in communications satellite systems to employ a broad-area beam antenna for uniformly illuminating the entire service area, such as the continental United States, with signal transmissions that operate in the entire allocated frequency band but with emissions that are either linearly or circularly polarized in a first direction and emissions that are oppositely polarized. Thus in the Ku-band, information-bearing carriers in the 11.7 to 12.2 GHz down-link frequency band can be vertically polarized and additional information-bearing carriers in the same frequency band can be horizontally polarized. Provided the oppositely polarized signals have approximately the same power flux density at the receiving antenna and the receiving earth station is properly equipped, the information content of the oppositely polarized, same frequency signals can be independently demodulated. Up-link communications can likewise be established in a manner analogous to the down-link communications. Polarization discrimination thus allows the allocated frequency band to be used twice in the same geographical area to effectively double information transfer capacity.
While the satellite architecture and communication link arrangements described above have been reasonably well suited for prior communication requirements, the total number of satellite positions available in the geostationary orbit and the allocated frequencies for up-link and down-link communications are limited. As the amount of world-wide signal traffic increases, a need arises for increasing the ultimate communications capacity of the satellite while reducing the on-board equipment and power requirements to provide a decrease in information transmission costs.